The present invention generally relates to the field of quantum processing devices and operations thereof, and more specifically, to operating quantum processing devices with fixed frequency quantum circuits (e.g., superconducting qubits), coupled to a frequency-tunable coupler, as well as related devices.
Recent advances in quantum enhanced sensing and quantum computing are making these technologies ever more relevant to industrial applications. Both quantum sensing and quantum computing make direct use of quantum-mechanical phenomena, such as superposition and entanglement. Quantum sensing aims at enhancing the precision of a measurement apparatus, whereas quantum computers perform operations on data entanglement. Superconducting circuits are relatively easy to manufacture with current technologies and are thus promising candidates to further scale quantum information technologies. Today, it can be envisioned that in the near term a small quantum computer, based on a couple of hundreds of superconducting qubits with limited to no error correction, will be able to simulate quantum systems intractable to conventional computers.
Despite improvements in engineering quantum systems, superconducting qubits can only store quantum information for a finite lifetime, which is known as the coherence time. Both quantum sensing and quantum computing ideally need a rapid and high-fidelity generation of highly entangled multi-qubit states. Currently, such states can only be prepared by sequentially carrying out many two-qubit gate operations, which preparation consumes a substantial amount of the coherence time of the qubits. Furthermore, this preparation limits the number of qubits that can be used, in practice, because the duration of the state preparation increases with the number of qubits. The duration of the state preparation must, however, be much shorter than the coherence time. When the duration of the state preparation is comparable to the coherence time, the system can no longer be operated. Shorter state preparation (as, for example, obtained using gates that act on and entangle multiple qubits at once) would allow a quantum computer or a quantum sensor to perform with less error and be scaled to a larger number of qubits.
In the field of superconducting qubits, the multi-qubit, single-step state preparation problem was so far not an issue. Indeed, the size of the studied systems is currently limited to a few qubits and reliable two-qubit entangling gates are short compared to the qubit's coherence time. There are known two-qubit gates, among various other possible existing realizations, which are implemented with transmon qubits, where qubits are coupled via a frequency tunable coupler element. In contrast to other approaches, this implementation takes advantage of the high coherence of fixed frequency qubits to generate entangled two qubit states with fidelities of more than 97%. However, gate rates are still relatively slow (hundreds of nanoseconds).